Conventionally, a variety of rotating electrical machines such as induction motor and permanent-magnetic synchronous motor have been proposed for use with power generators. Typically, each rotating electrical machine has windings and a core so that magnetic fluxes generated by an application of electric current to the windings are concentrated in the core to increase the intensity of magnetic field and a resultant rotational force of the rotating electrical machine.
For this purpose, the core is made of ferromagnetic material such as iron and has slots or grooves defined therein for accommodation of the windings. The core is made by stacking a plurality of thin plates each covered by a suitable electrically insulating material. In order to reduce eddy current caused by the change of magnetic flux generated by the windings or the permanent magnets, the plates are made from alloy steel such as magnetic steel sheet with reduced magnetic hysteresis and elevated saturation magnetization.
Preferably, the rotating electrical machine has elevated power conversion efficiency. To satisfy this requirement, copper loss and eddy current loss should be minimized. For example, the copper loss results in an electric resistance heating due to electric current which flows in the conductive members such as armature or stator windings. In particular, the copper loss occupies a large part of the total loss in the rotating devices such as small- or medium-size, brushless rotating devices or permanent magnet synchronous generators.
The eddy loss results in the electric resistance heating due to the eddy current generated around the magnetic flux, which is in proportion to the square of the driving frequency of the rotating device. Then, an increase of the driving frequency as well as the resultant rotating frequency causes a significant increase of the eddy current in the windings.
This means that, in order to decrease the copper loss, it is important to decrease the electric resistance in the armature or stator windings. In line with this, JP 2000-217282 A discloses to increase a ratio of the gross cross-sectional area of the windings relative to the cross-sectional area of the slots or grooves for receiving the windings and thereby decrease the copper loss to be generated in the stator.
Conventionally, it has been known to form a bundle of windings by bundling a plurality of circular or rectangular cross-section wires. Using the bundle of windings can decrease more eddy current loss than using large-diameter windings. The bundle of windings is made simply by bundling a number of parallel wires. JP 2006-325338 A discloses to a twisted winding made by twisting a number of wires.
Recently, a superconducting rotating electrical machine is proposed in JP 2005-176578 A, for example, in which superconducting property is provided to the rotor and the stator by forming the armature windings using the superconducting wires for the purpose of increasing the power conversion efficiency and the downsizing of the superconducting rotating electrical machine. JP 2005-176578 A also discloses a superconducting rotating electrical machine incorporating superconducting rotor and stator, in which the windings are made of superconducting material to introduce the superconducting property and thereby a compact rotating electric machine with an increased electric conversion efficiency can be obtained.
In the meantime, a size of the normal-conducting rotating electrical machine with a stator or rotor made of ferromagnetic material such as iron is substantially determined by a saturated magnetic field intensity, or a cross-sectional area of the ferromagnetic material or a number of windings required for obtaining a predetermined output. Also, as described above, the eddy current loss in the rotating electrical machine can be reduced by stacking thin plates made of material exhibiting smaller iron loss such as magnetic steel and covered by electrically insulating material.
Typically, a ratio of the gross cross-sectional area of the windings relative to the total cross-sectional area of the slot in the normal-conducting rotating electrical machine ranges from about 30% to about 40%, which is insufficient for effectively reducing a temperature increase due to the copper loss in the stator if a large amount of electric current is applied to the windings.
Also, although the bundle of windings with circular or rectangular cross-sectional wires may be used to ease the winding operations, the number of windings relevant to the eddy current loss is limited to about 20 to 30.
Further, using stator windings made of bismuth-based high temperature superconducting wires typically employed in the superconducting rotating electrical machine may affect the superconducting property such as an increase in temperature of the windings due to heat generated by the alternating-current loss and a resultant decrease in critical current. Then, providing a superconducting property for the stator may be technically difficult and, even if possible, result in a cost increase in the manufacturing of the superconducting rotating electrical machine. Also, forming the stator windings of the superconducting rotating electrical machine with bismuth-based high temperature superconducting material conventionally used in the superconducting rotating electrical machine results in the temperature increase in the windings due to alternating-current loss and thereby reduces the critical current associated with the superconducting property of the windings. In conclusion, it may be technically impossible to provide a superconducting property for the stator. Also, providing a superconducting property for the stator will result in a cost increase in the manufacturing of the superconducting rotating electrical machine.
In the meantime, a superconducting rotating electrical machine with a superconducting rotor and a normal-conductive stator can increase the intensity of magnetic field and, as a result, decrease the overall manufacturing cost of the superconducting rotating electrical machine, which reduces copper loss in the superconducting rotor and, as a result, increases the power conversion efficiency, but increases copper loss, iron loss, or eddy current loss in the normal-conductive stator. Also, the magnetic field of the iron core is easy to be saturated by the elevated magnetic field from the armature, which prohibits an application of alternating filed. This results in that the elevated efficiency derived from the superconducting rotor is cancelled by the losses in the stator, which fails to meet the requirements for the further increase of power conversion efficiency in the superconducting rotating electrical machine. To address this problem, the superconducting rotor and the normal-conductive stator may be combined with each other to increase the intensity of magnetic field and thereby prevent a significant increase of the manufacturing cost. This also increases the power conversion efficiency of the rotating electrical machine. As a tradeoff, however, the normal-conductive property in the stator causes copper loss, iron loss, and/or eddy current loss. Also, the magnetic field of the iron core will be instantly saturated by the magnetic filed of the armature, which weakens the alternating magnetic field. Therefore, the elevated efficiency derived from the superconducting—rotor will be cancelled by the losses in the stator.
Accordingly, the present invention is to solve those problems and an object of the invention is to increase the power conversion efficiency in the superconducting rotating electrical machine with the superconducting rotor and normal-conductive stator.